The present invention relates to a voltage controlled oscillator (VCO), and more particularly to a circuit for a VCO suitable for manufacture as a monolithic integrated circuit.
Voltage controlled oscillators have many applications in generating an oscillating voltage whose frequency is dependent upon a control voltage. One application of a voltage controlled oscillator is in a phase locked loop (PLL) in which the output of the oscillator is locked to an input frequency and the control voltage of the VCO is used as a reference voltage for other circuits. For example, it may be desirable to adjust the frequency response of a filter circuit as a function of the input frequency.
U.S. Pat. No. 5,243,239 discloses an integrated MOSFET oscillator using storage cells to effect trimming of current and voltage. A constant current oscillator is disclosed which uses a capacitor charged by a constant current as a timing element, with the capacitor being discharged by a MOSFET connected across it. The MOSFET is used as a switch to discharge the capacitor, and the frequency of oscillation is not substantially dependent on its linear resistance.
It would be desirable to provide a voltage controlled oscillator whose input frequency is not substantially affected by variations in supply voltage. It would also be desirable to provide a voltage controlled oscillator whose frequency is substantially dependent on a control voltage applied to a timing element, preferably a grounded timing element, where a similar timing element could be incorporated in another circuit, for example a filter. The VCO could be incorporated in an integrated circuit having several timing elements in filters. The output of the VCO can be locked to a reference frequency, e.g., using a phase locked-loop (PLL), and the tuning voltage used to tune the filters, e.g. continuous time filters. Such an integrated circuit could be used in an Ethernet transceiver.